The Effects of Endotracheal Tube and i-gel® Supraglottic Airway Device on Respiratory Impedance: A Prospective Observational Study

The forced oscillation technique (FOT) is a non-invasive method of measuring respiratory impedance, which is defined as the spectral relationship between pressure and airflow (1). Impedance is determined by two components: respiratory resistance (Rrs) and respiratory reactance (Xrs); the latter reflects the elastic and inertial properties of the lung. FOT measurement requires no special breathing maneuvers or interference with normal breathing (2). The clinical use of FOT has progressed as more FOT devices have become commercially available, such as the MostGraph-01® impulse oscillation system (Chest MI, Tokyo, Japan) (3). The evidence base for the clinical utility of FOT has expanded, especially for the evaluation and management of obstructive pulmonary diseases. Nonetheless, the changes in respiratory impedance that occur as a result of airway inflammation and pulmonary function are not fully understood. The Rrs measured at 5 Hz (R5) and 20 Hz (R20) are representative of low and high frequency resistances, respectively. The difference between R5 and R20 (R5 - R20) is referred to as the frequency dependence of Rrs. The reactance at 5 Hz (X5) reflects the combined effects of tissue elastance and inertia, although at this lower frequency, the effect of tissue elastance would be predominant. Therefore, X5 reflects the elastic recoil of the peripheral airways. The resonant frequency (Fres) is the frequency at which respiratory reactance equals 0 cmH₂O/L/s. The Fres marks the transition from capacitative dominance at low frequencies to inertial dominance at high frequencies. The area of low reactance (ALX) is the area created by three lines (those of frequency 5 Hz, Xrs = 0 cmH₂O/L/s, and the Xrs curve). The ALX is a useful index associated with respiratory compliance and therefore, reflects the patency of the small airways. The ALX is a single quantity that reflects changes in the degree of peripheral airway obstruction and closely correlates with R5 - R20 (4). We examined the changes in respiratory impedance after endotracheal intubation or the use of a supraglottic airway device (SGD) in mechanically ventilated patients. We hypothesized that endotracheal intubation would bring about more substantial preoperative increases in respiratory resistance and reactance than using SGD.

This prospective study was approved by the ethics committee of Osaka Medical College, Japan (approval reference number 1252) and conducted in accordance with the declaration of Helsinki (1964). Written informed consent was obtained from all participants. The study was registered at the Japan medical association center for clinical trials on September 2nd, 2013 (reference JMA-IIA00136). The principal investigator was Junko Nakahira. We enrolled 40 patients with American Society of Anesthesiologists physical status classification 1 or 2 undergoing general anesthesia for transurethral resection of bladder tumor in a 5-month period in 2014. Potential participants underwent spirometry without bronchodilation. We excluded those with a history or symptoms of asthma, such as coughing or wheezing at rest, patients diagnosed with chronic obstructive pulmonary disease according to the global initiative for chronic obstructive lung disease guidelines (5), and patients who had taken oral steroids or had had respiratory tract infection or exacerbation within the previous 3 months.

Spirometry and FOT were performed the day before surgery. On the day of surgery, anesthesia was induced with intravenous propofol 2 mg/kg, rocuronium 0.8 mg/kg and an infusion of remifentanil at 0.5 μg/kg/min. The urologist determined the need for neuromuscular blockade during surgery according to the location of the tumor. Neuromuscular blockade was applied to patients with tumors located on the lateral bladder wall to avoid stimulation of the obturator nerve. The application of neuromuscular blockade was not needed for patients with tumors located at other sites. A cuffed endotracheal tube (Portex Soft Seal®, Smiths Medical, Kent, UK) of internal diameter 7.0 mm for women and 8.0 mm for men was placed in patients who needed a neuromuscular blocker; a supraglottic device (i-gel®, Intersurgical, Wokingham, UK) of size 3 for women or size 4 for men was inserted in the remainder. Anesthesia was maintained with inhaled sevoflurane 1.0% - 1.5% and intravenous remifentanil 0.25 - 0.5 μg/kg/min in a fraction of inspired oxygen (FiO₂) of 0.4. All patients in both groups were mechanically ventilated with volume controlled ventilation at 8 mL/kg predicted body weight without positive end-expiratory pressure. For postoperative analgesia, acetaminophen 1,000 mg was administered intravenously at the end of the surgery. Anesthetic agents were stopped immediately after the operation. After patients were breathing spontaneously, the FiO₂ was increased to 1.0, and sugammadex 1.5 mg/kg was administered intravenously to those who had received rocuronium. Extubation or removal of the supraglottic device occurred when the patients responded to their name, body temperature was > 35.5°C, peripheral oxygen saturation was > 97%, and their breathing became stable at a rate of 10 - 20/min. In those whose tracheas had been intubated, open suctioning with 14-Fr catheter at -20 kPa was performed through the endotracheal tube to remove sputum, but the catheter tip was not advanced into the bronchus. After the removal of the airway adjunct, oxygen was administered by facemask at a rate of 4 L/min for 4 hours. All participants were encouraged to walk on the first postoperative day.

Forced oscillation was measured according to standard techniques using a MostGraph-01 device (6, 7) the day before surgery and after removal of the airway adjunct. The measurements of postoperative respiratory impedance were taken after the patients were extubated while lying on the operation table and before they were sent back to their ward. Rrs and Xrs were recorded in patients in the sitting position under normal breathing conditions through a mouthpiece while they were wearing a nose clip. To minimize artifacts from vibrations, an investigator supported the patient’s cheeks. For preoperative measurements, patients sat unsupported on a chair. Postoperative measurements were recorded on the operating table with the patients sitting at 45 - 50° with their legs straight, before they were sent back to the ward.

Of the 40 participants, the trachea was intubated in 23 (ET group) and an SGD was used in the remaining 17 (SGD group). There was no significant between-group difference in demographic or preoperative clinical characteristics (Table 1). None of the patients required endotracheal suction during mechanical ventilation, and patient-ventilator asynchrony was not observed. Endotracheal suction was performed in all patients in the ET group just before extubation. None of the patients needed a bronchodilator or expectorant agent, and none had intra- or post-operative respiratory failure or airway complications. Three patients in whom an SGD was used ultimately required endotracheal intubation.

There were no significant differences in preoperative FOT parameters between the groups. Postoperatively, R5, R20, R5-R20, and ALX were significantly higher than baseline in the ET group (P = 0.002, 0.004, 0.015, and 0.035, respectively with Student’s t-test with unequal variances, P = 0.002, 0.002, 0.025, and 0.019 with ANCOVA; Table 2). In each group, postoperative R5, R20, R5-R20, Fres, and ALX increased significantly, and X5 decreased significantly (P < 0.005 for all). Preoperative changes in all FOT parameters of respiratory resistance and reactance were significantly greater in the ET group than the SGD group.

We found that endotracheal intubation had a greater influence on Rrs (R5, R20, and R5-R20) and ALX than using supraglottic airway device. Although the normal limits for Rrs have not been established, Kurosawa suggested that R5 >3 cmH₂O/L/s is abnormally high (8). The abnormally increased Rrs, which we detected after airway management including mechanical ventilation and extubation, might reflect narrowing or obstruction of the trachea, bronchi or bronchioles. Generally, edema of the vocal folds does not occur when an SGD is used, and tracheal suctioning is not required. The SGD, therefore, would not be expected to adversely influence the anatomy or function of the trachea or bronchi. Other possible explanations for the significant postoperative deterioration in all FOT parameters are proximal and peripheral airway edema caused by excessive infusion of intravenous fluids or the lithotomy position, and sputum-, asthma- and/or ventilator-induced lung injury. The present study does not allow us to differentiate between these possible causes.

Respiratory reactance generally reflects parenchymal integrity, but may be difficult to interpret. Xrs reflects the elastic and inertial properties of the lung, while Fres and ALX are somewhat abstract parameters. The normal range for Fres is approximately 6 - 11 Hz, and is generally < 0.33 kPa/L (3.37 cmH₂O/L) for ALX (9). In this study, preoperative changes in all FOT parameters were influenced by anesthesia to a certain extent, independent of airway adjunct.

It has been reported that preoperative FOT measurements and spirometry parameters improve in children with cystic fibrosis undergoing general anesthesia breathing spontaneously through an SGD for insertion of a peripheral central venous catheter (10). Given these findings, we believe that spontaneous breathing likely explains these improvements. We use remifentanil routinely, so spontaneous breathing tends not to occur in the SGD group. It seems likely that spontaneous breathing through an SGD is the best anesthetic strategy for patients with severe respiratory disease, although further research is needed.

Our study had several limitations. First, it was not randomized; patients were allocated to each group on the basis of surgical need for neuromuscular blockade. Second, perioperative fluid balance might have influenced the outcomes, but it was not possible to make an accurate assessment of fluid input due to the use of bladder irrigation fluid during transurethral resection. Finally, our findings may have been influenced by differences in the proportions of smokers between the groups, or differences in other unmeasured variables, such as the requirement for intraoperative tracheal suctioning, postoperative pain, and difficulty in holding the mouthpiece or discomfort caused by a urethral catheter.